FIELD: The subject invention is in the field of mechanical power transmission and transfer mechanisms, particularly the field of power transfer from an engine in a boat to the propeller driven by the engine. More particularly it is in the field of such drives in which the drive shaft and propeller shaft are parallel and essentially horizontal such as in well known inboard/outboard drives in which power is transmitted through the stem of a boat above the waterline and then down to the propeller shaft and propeller. However, the subject drive is an inboard drive which transmits power through the bottom of the boat. A feature common to inboard/outboard drives and similar inboard drives is that the drive shaft and propeller shaft are parallel and power is transmitted between the two using bevel and/or miter gearing, chains or belts. One important objective of such drives is that the components in the water present as low frontal area as possible to minimize drag losses. This is particularly important for sailboats in which the propeller is an auxiliary power source and must present minimum drag when the boat is under sail. The problem is more severe for larger boats in which auxiliary power levels are in the range of 100 to 200 H.P. Since such boats are not high speed boats, propeller speeds must be relatively low and propellers fairly large to achieve satisfactory efficiency. These facts require that the torque capacity of the drive be high relative to the horsepower level. In the stated horsepower range high torque per horsepower gear drives become bulky and require undesirably large frontal areas under water and, for assembly reasons, their casings comprise several parts, in many cases having long parting lines. Chain drives are better suited to high torque per horsepower transmissions; however good operation, efficiency and long life of chain drives, particularly bi-directional drives, requires that the chains be under tension and correspondingly free of slack and running in a straight line from sprocket to sprocket. It is close to physically impossible and economically and practically impossible, using conventional techniques, to design and assemble a chain drive in which the chain is always in tension without using some sort of tensioning device. This is caused in large part by the fact that use invariably involves wearing in and wear which allow the chain(s) to go slack. Tensioning devices inherently tend to add undesirable amounts of frontal area and complication.
PRIOR ART: There is much prior art in the particular field described above and many of the prior art drives use chains. The U.S. patents listed here are typical examples:
1. 2,809,605
2. 3,403,655
3. 3,795,219
4. 3,951,096
5. 4,645,463
6. 4,869,692
7. 4,887.983
8. 4,925.413
9. 4,932,907
10. 4,992,066
11. 5,813,887
12. 5,961,358
13. 6,413,127
As background to discussion of this prior art, it is important to state that the chains having the highest power capacity for their size and weight are chains known as silent chains. These chains comprise pluralities of flat links having a tooth form at each end. The side-by-side links are pinned end to end so that the teeth forms form teeth when the chain is in contact with the sprockets on which it is mounted. Belt width is determined by the numbers of links pinned side-by-side. Making and using these chains as mechanically efficiently as possible resulted in their having the characteristics that (1) they allow only limited bending in the direction away from the toothed side of the belt, and (2) the durability and efficiency depend on their being as straight as possible between sprockets at all times. These factors relate to the need for tension adjustment and prohibit techniques using tension idlers which would not allow the chain to be straight between sprockets.
Regarding the cited prior art, patents 2, 5, 6, 7, 8, 9 and 10 utilize roller chains and show no specific means for adjusting tension except in patents 1 and 10. The adjustment in patent 1 is accomplished by adjusting the distance between the drive and driven shafts by having the shafts in separate assemblies which fit together telescopically. It is judged that making the housings telescopic is not an economically efficient technique and is mechanically cumbersome. The means shown in patent 10 comprises an oval shaped cam pivotally mounted midway between the lengths of chain between sprockets so that rotating the cam in one direction so that the cam ends contact the chain lengths spreads them apart to increase tension. This does not allow the chain in tension to be straight. Also, this means can only be used in unidirectional drives. With rotation in the wrong direction the cam would be forcefully rotated into the chain and jammed.
The remaining patents show drives using belts of some kind. Patent 2 uses a toothed belt and shows no means of adjusting tension even for the purpose of removing and installing the belt. Patent 3 also shows the use of a toothed belt with means for adjusting tension for the purpose of removing and installing the belt but none for compensating for belt stretch and other factors which are known to cause loosening of toothed belts. The means used comprise a spherically mounted bearing on one end of the drive shaft so that when the housing is disassembled and the bearing at the other end of the drive shaft is removed, the shaft can drop to an angle sufficient to allow the teeth on the belt to clear the rim on the sprocket, thus facilitating removal and replacement. The end of the shaft freed by dismantling the casing is tapered to facilitate its reentry into the bearing when the case is assembled, leveling the drive shaft again and providing nominally acceptable belt tension. However, this adjustment feature does not allow compensation for belt stretch and other factors which are known to cause belt loosening. Also, to enable replacement of this belt the casing is divided vertically into forward and aft parts, generating a need for long parting surfaces and a plurality of fasteners are needed to assemble the casing.
Patent 4) shows the use of dual toothed belts. There are no provisions for tension adjustment and the method of assembly and disassembly of the belt drive is not disclosed.
Many motorcycles transmit driving power from a drive shaft to a driven shaft, the rear axle. Chain tension is adjusted by adjusting the position of the rear axle and everything carried by it relative to the drive shaft. This technique cannot be used in propeller drives because the driven shaft must be enclosed in a housing.
The closest prior art for the subject invention is disclosed in U.S. patent application Ser. No. 09/883,455, filed Jun. 18, 2001, now allowed and titled: xe2x80x9cLow Frontal Area, Inboard Thru-Hull Propeller Drive and Methods For Assembling and Adjusting the Drivexe2x80x9d and invented by the inventor of the subject invention. In this drive the driving shaft is in one subassembly and the driven propeller shaft in another subassembly. Chain tension is adjusted by adjusting the space between the two subassemblies and sealing means are provided to seal the gap between the assemblies while accommodating the variation in gap width.
In view of this prior art, the objective of the subject invention is to provide a low frontal area, inboard, through-hull propeller drive for power ranging up to 200 H.P. at maximum propeller shaft speeds of 2500 RPM, the drive using a silent chain and having (1) a housing having a minimum number of parts and short parting lines, (2) simple means for adjusting chain tension for installation and removal purposes and compensation for wear, (3) allowing simple chain installation and removal, and (4) using simple conventional scaling techniques.
The subject invention is a low frontal area, inboard, through-hull propeller drive. The drive comprises (1) a lower subassembly having a casing having a high fineness ratio streamlined cross section shape strut and a propeller shaft, chain sprocket and bearings installed in its lower end through the opening for the propeller shaft and its bearings, (2) an upper subassembly attached to the upper end of the lower subassembly, having a drive shaft, sprocket and bearings installed in it, and (3) a silent chain interconnecting the sprockets, which are of the same diameter, in the lower and upper subassemblies
The drive shaft and sprocket are carried in the upper assembly on two bearings, each of which is carried in a bearing carrier adjustably attached to the exterior of an end of the drive casing. The bearing carriers are structurally interconnected and chain tension is adjusted by adjusting the interconnected carriers up and down on the drive casing. Sealing is provided by face seals, such as O-rings, between the surfaces of the bearing carriers and the ends of the casing.
The assembly procedure is to install the chain in the lower subassembly and attach the casing of the upper subassembly to the upper end of the lower assembly with the chain ends extending into the casing. The drive shaft and sprocket are inserted into the casing and the chain ends are connected using the slack available because the bearings and carriers are not yet installed. Then the bearings and carriers are installed and the carriers are structurally interconnected to coordinate their adjustment movement relative to the casing. The chain tension is adjusted by moving the shaft, sprocket and bearings toward and away from the lower assembly using threaded means acting between the shaft, sprocket, bearing carriers, bearings and the interconnecting structure and the top of the casing.